The invention relates in general to the field of liquid crystal display design and, more particularly, to the fabrication of organic O-plate compensator elements. Specifically, the invention describes an O-plate compensator incorporating polyimide materials having bulky side-chain groups.
Liquid crystals are useful for electronic displays because polarized light traveling through a liquid crystal layer is affected by the layer""s birefringence, which can be changed by the application of a voltage across the layer. By using this effect, the transmission or reflection of light from an external source can be controlled with much less power than is required for the luminescent materials used in other types of displays. As a result, liquid crystal displays (LCDs) are now commonly used in a wide variety of applications, such as, for example, digital watches, calculators, portable computers, and many other types of electronic equipment. These applications highlight some of the advantages of LCD technology including very long operational life in combination with very low weight and low power consumption.
The information content in many LCI)s is presented in the form of multiple rows of numerals or characters, which are generated by segmented electrodes deposited in a pattern on the display. The electrode segments are connected by individual leads to electronic driving circuitry. By applying a voltage to the appropriate combination of segments, the electronic driving circuitry controls the light transmitted through the segments.
Graphic and television displays may be achieved by employing a matrix of pixels in the display which are connected by an X-Y sequential addressing scheme between two sets of perpendicular conductors. More advanced addressing schemes, applied predominantly to twisted nematic LCDs, use arrays of thin film transistors to control driving voltages at individual pixels.
Contrast and stability of relative gray scale intensities are important attributes in determining the quality of a LCD. The primary factor limiting the contrast achievable in a LCD is the amount of light which leaks through the display in the dark state. In addition, the contrast ratio of the LCD also depends on the user""s viewing angle. The contrast ratio in a typical LCD is a maximum only within a narrow viewing angle centered near normal incidence and drops off as the angle of view is increased. This loss of contrast ratio is caused by light leaking through the black state pixel elements at large viewing angles. In color LCDs, such leakage can also cause severe color shifts for both saturated and gray scale colors.
The viewing zone of acceptable gray scale stability in a typical prior art twisted nematic LCD is severely limited because, in addition to color shifts caused by dark state leakage, the optical anisotropy of the liquid crystal molecules results in large variations in gray level transmission as a function of viewing angle. The variation is often severe enough that, at extreme vertical angles, some of the gray levels reverse their transmission levels. These limitations are particularly important for applications requiring a very high quality display, such as in avionics, where viewing of cockpit displays from both pilot and copilot seating positions is important. Such high information content displays require that the relative gray level transmission be as invariant as possible with respect to viewing angle. It would be a significant improvement in the art to provide a liquid crystal display capable of presenting a high quality, high contrast image over a wide field of view.
FIGS. 1A and 1B show a conventional normally white, twisted nematic LCD 100 including a polarizer 105, an analyzer 110 with a polarization axis perpendicular to that of the polarizer 105, a light source 130, and a viewer 135. (The polarizer 105 and the analyzer 110 both polarize electromagnetic fields. Typically, however, the term xe2x80x98polarizerxe2x80x99 refers to a polarizer element that is closest to the source of light while the term xe2x80x98analyzerxe2x80x99 refers to a polarizer element that is closest to the viewer of the LCD.) In the normally white configuration of FIGS. 1A and 1B, a xe2x80x9cnonselectxe2x80x9d area 115 (no applied voltage) appears light, while a xe2x80x9cselectxe2x80x9d area 120 (those which are energized by an applied voltage) appear dark. In the select area 120 the liquid crystal molecules tend to tilt and rotate toward alignment with the applied electric field. If this alignment were perfect, all the liquid crystal molecules in the cell would be oriented with their long axes normal to the cell""s major surface. This configuration is known as homeotropic alignment.
Because the liquid crystals used for twisted nematic displays exhibit positive birefringence, this arrangement, known as the homeotropic configuration, would exhibit the optical symmetry of a positively birefringent C-plate. As is well known in the art, a C-plate is a uniaxial birefringent compensator with its extraordinary axis (i.e., its optic or c-axis) perpendicular to the surface of the plate (parallel to the direction of normally incident light). In the select state the liquid crystal in a normally white display would thus appear isotropic to normally incident light, which would be blocked by the crossed polarizers.
One reason for the loss of contrast with increased viewing angle which occurs in a normally white display is that a homeotropic liquid crystal layer will not appear isotropic to off-normal light. Light propagating through the liquid crystal layer at off-normal angles appears in two modes due to the birefringence of the layer; a phase delay is introduced between those modes and increases with the incident angle of the light. This phase dependence on angle of incidence introduces an ellipticity to the polarization state which is incompletely extinguished by the second polarizer, giving rise to light leakage. To correct for this effect, an optical compensating element must also have C-plate symmetry, but with negative birefringence (ne less than no). Such a compensator will introduce a phase delay opposite in sign to the phase delay caused by the liquid crystal layer, thereby restoring the original polarization state and allowing light passing through energized areas of the layer to be blocked more completely by the output polarizer. C-plate compensation, in general, does not impact the variation of gray scale with viewing angle which is addressed by the present invention.
FIG. 2 depicts the coordinate system which is used to describe the orientation of both liquid crystal and birefringent compensator optic axes. Light propagates toward the viewer 200 in the positive z direction 205 which, together with the x-axis 210 and the y-axis 215, form a right-handed coordinate system. Backlighting is provided, as indicated by arrows 220. The polar tilt angle xcex8 225, also referred to as a pretilt angle, is defined as the angle between the liquid crystal""s molecular optic axis ĉ 230 and the x-y plane, as measured from the x-y plane. The azimuthal or twist angle xcfx86 235 is measured from the x-axis to the projection 240 of the optic axis onto the x-y plane.
Normally White Twisted Nematic LCDs
FIG. 3 is a cross sectional schematic view of a prior art twisted nematic, transmissive type. normally white liquid crystal display. The display includes a polarizer layer 300 and an analyzer layer 305, between which is positioned a liquid crystal layer 310, consisting of a liquid crystal material in the nematic phase.
It is convenient in describing the orientation of various compensation elements of the display to refer to a normal axis perpendicular to the display, which is depicted by a dashed line 370. In the case of a normally white LCD, the polarizer 300 (with a polarization direction in the plane of the drawing 315) and the analyzer 305 (with a polarization direction 320 perpendicular to the plane of the drawing) are oriented with their polarization directions at 90xc2x0 to one another. A first transparent electrode 325 and a second transparent electrode 330 are positioned on the glass plates 340 and 345 adjacent to opposite surfaces of the liquid crystal layer 310 so that a voltage can be applied, by means of a voltage source 335, across the liquid crystal layer. As is explained below, the inner surfaces of the glass plates 340 and 345, which are proximate to the liquid crystal layer 310, can be physically or chemically treated to affect the desired liquid crystal orientation, as by buffing.
As is well known in the LCD art (see, e.g., Kahn, xe2x80x9cThe Molecular Physics of Liquid Crystal Devices,xe2x80x9d Physics Today, pp. 66-74, May 1982), when the inner surfaces of the plates 340 and 345 (the surfaces adjacent to the liquid crystal layer 310) are coated with a surface treatment for aligning the liquid crystal, such as a polyimide, buffed, and oriented with their buffed directions perpendicular, the director of the liquid crystal material, absent any applied electrical voltage, will tend to align with the buffed direction (known as the xe2x80x9crub directionxe2x80x9d) in the regions of the layer 310 proximate each of the plates 340 and 345. Furthermore, the orientation of the liquid crystal axis (i.e., the director) will twist smoothly with respect to the normal axis through an angle of approximately 90xc2x0 along a path in the layer 310 from a first major surface adjacent to the plate 340 (i.e., at the 310/340 interface) to a second major surface adjacent to the plate 345 (i.e., at the 310/345 interface).
In the absence of an applied electric field the direction of incoming polarized light will be rotated by 90xc2x0 in traveling through the liquid crystal layer 310. When the glass plates and the liquid crystal layer are placed between crossed polarizers, such as the polarizer 300 and the analyzer 305, light polarized by the polarizer and traversing the display, as exemplified by the light ray 350, will thus be aligned with the polarization direction of the analyzer 320 and therefore will pass through the analyzer.
When a sufficient voltage is applied to the electrodes 325 and 330, however, the applied electric field causes the director of the liquid crystal material to tend to align parallel to the field. With the liquid crystal material in this state, light passed by the polarizer 300, as illustrated by the light ray 355, will be extinguished by the analyzer 305. Thus, an energized pair of electrodes will produce a dark region in the display, while light passing through regions of the display which are not subject to an applied field will produce illuminated regions. As is well known in the art, an appropriate pattern of electrodes, activated in selected combinations, can be utilized in this manner to display alphanumeric or graphic information. As explained further below, one or more compensator layers, such as the layers 360 and 365, may be included in the display to improve the quality of the display.
O-Plate Gray Scale Compensation
To substantially eliminate reversal of gray levels and improve gray scale stability, a birefringent O-plate compensator can be used. The O-plate compensator, as described in U.S. Pat. No. 5,504,603, utilizes a positive birefringent material with its principal optic axis oriented at a substantially oblique angle with respect to the plane of the display (hence the term xe2x80x9cO-platexe2x80x9d). xe2x80x9cSubstantially obliquexe2x80x9d implies an angle appreciably greater than 25xc2x0 and less than 65xc2x0. O-plates have been utilized, for example, with angles relative to the plane of the display between 30xc2x0 and 60xc2x0, typically at 45xc2x0. Moreover, O-plates with either uniaxial or biaxial materials can be used. O-plate compensators can be placed in a variety of locations between a LCD""s polarizer layer and analyzer layer.
In general, compensation schemes using O-plate compensators may also include A-plates and/or negative C-plates. As is well known in the art, an A-plate is a birefringent layer with its extraordinary axis (i.e., its c-axis) oriented parallel to the surface of the layer. Its a-axis is thus oriented normal to the surface (parallel to the direction of normally incident light), leading to its designation as an A-plate. A-plates may be fabricated by the use of uniaxially stretched polymer films, such as polyvinyl alcohol, or other suitably oriented organic birefringent materials.
A C-plate is a uniaxial birefringent layer with its extraordinary axis oriented perpendicular to the surface of the layer (parallel to the direction of normally incident light). Negatively birefringent C-plates may be fabricated by the use of uniaxially compressed polymers (See, e.g., Clerc et al., U.S. Pat. No. 4,701,028), stretched polymer films, or by the use of physical vapor deposited inorganic thin films (See, e.g., Yeh et al., U.S. Pat. No. 5,196,953).
Oblique deposition of a thin film by physical vapor deposition is known to produce a film having birefringent properties (see, e.g., Motohiro and Taga, xe2x80x9cThin Film Retardation Plate by Oblique Deposition,xe2x80x9d Appl. Opt.,. Vol. 28, No. 3, pp. 2466-2482, 1989). By further exploiting the tilted orientation of the symmetry axis, the Motohiro process can be refined or enhanced to fabricate O-plates. Such components are by their nature biaxial. Their growth characteristics generate a microscopic columnar structure. The angles of the columns are tipped toward the direction of the arrival of a vapor stream. A deposition angle (measured from normal) of 76xc2x0, for example, results in a column angle of approximately 45xc2x0. The columns develop an elliptical cross section as the result of shadowing. This elliptical cross section gives rise to the biaxial character of the films. The birefringence, in magnitude and symmetry, is entirely attributable to the film microstructure and is referred to as form birefringence. These phenomena in thin films have been extensively studied and described by Macleod (xe2x80x9cStructure-Related Optical Properties of Thin Films,xe2x80x9d J. Vac. Sci. Technol. A, Vol. 4, No. 3, pp. 418-422, 1986).
Uniaxial O-plate components can also be used to improve gray scale stability in twisted nematic liquid crystal displays. These may be fabricated by the use of suitably oriented organic birefringent materials. Those skilled in the art will recognize other means for fabricating both uniaxial and biaxial O-plates.
Elimination of gray scale reversal by the use of an O-plate compensator layer occurs in the following manner. In the positive vertical viewing direction, the retardation of the O-plate increases with viewing angle and tends to offset the decreasing retardation of the liquid crystal layer. When the viewer is looking down the axis of the average liquid crystal director, the presence of the O-plate prevents the layers between the two polarizers from appearing isotropic.
In the negative vertical viewing direction, the combination of an O-plate and an A-plate with their optic axes nominally at right angles tends to exhibit birefringence characteristics similar to that of a negative birefringence retarder with its optic axis oriented perpendicular to the plane containing the axes of the O-plate and A-plated. The direction of this retarder axis is nominally parallel to the orientation of the average liquid crystal in the central region of the cell when it is driven at a voltage between select and non-select states. Thus, the presence of an O-plate oriented in this manner tends to cancel the birefringence of the liquid crystal layer. A similar effect occurs in the positive and negative horizontal viewing directions.
The orientations of a compensator""s optic axes can be carefully chosen so that the combined retardation effects cancel each other in the normal incidence viewing direction as well as improving viewing in the horizontal direction. Combinations of more than one O-plate can be used as long as their orientations satisfy these requirements. Furthermore, negative C-plates can, for certain configurations, increase the contrast ratio at large fields of view, occasionally with some decrease in gray scale stability.
O-Plate Technology
The liquid crystal layer, the compensator layer(s), the polarizer layer, and the analyzer layer may assume a variety of orientations relative to one another in a liquid crystal display. Some of the possible configurations are shown in Table 1, where xe2x80x98Axe2x80x99 represents an A-plate, xe2x80x98Cxe2x80x99 represents a C-plate, xe2x80x98Oxe2x80x99 represents an O-plate, xe2x80x98LCxe2x80x99 represents the liquid crystal, and xe2x80x98OxOxe2x80x99 represents crossed O-plates. Crossed O-plates are adjacent O-plates with their azimuthal angles xcfx86 235 nominally crossed, one oriented between 0xc2x0 and 90xc2x0 and the second oriented between 90xc2x0 and 180xc2x0.
The projections of the principal axes onto the plane of the display with respect to the liquid crystal director orientation can vary with the embodiment. In some cases, for example with two O-plates, the O-plate axis projections are at approximately 45xc2x0 with respect to the average liquid crystal director orientation near the center of the liquid crystal cell, while in others, the O plate axis projection is substantially parallel to that of the liquid crystal director.
Crossed O-plate (OxO) designs that are further compensated with A-plates provide additional design flexibility. The choice of A-plate value is not critical as such designs can be adjusted by varying the relative orientations of the A-plates. Thus, it is possible to generate desired solutions with commercially available A-plate retardation values.
The flexibility which an O-plate compensation scheme offers the display designer allows tailoring of performance to specific display product requirements. It is possible, for example, with a simple configuration and parameter modifications to achieve an isocontrast optimized for left or right viewing, an isocontrast optimized for extreme vertical angle viewing, or an isocontrast optimized for viewing at both large left and right angles above normal viewing. It is also possible to adjust the configuration and parameters to improve both the contrast and gray scale stability over a specified field of view, or to further optimize one at the expense of the other.
When viewed at an angle near the normal to its surface, a twisted nematic liquid crystal display provides high quality output. At large viewing angles, however, the image tends to degrade and exhibit poor contrast and gray scale uniformity. This occurs because the phase retardation effect of the liquid crystal material on light passing through it inherently varies with the inclination angle of the light, leading to a lower quality image at large viewing angles. By introducing one or more optical compensating elements in conjunction with the liquid crystal cell, it is possible to substantially correct for the undesirable angular effects and thereby maintain higher contrast and stable relative gray scale intensities at larger viewing angles than otherwise possible.
A liquid crystal display using a positively birefringent O-plate compensator is described herein that makes possible a significant improvement in the gray scale properties and contrast ratios of liquid crystal displays over a wide range of viewing angles. Further, the described method can reduce the cost of manufacturing an O-plate compensator by reducing the number of thin film layers typically used in its construction.
An O-plate compensator (oblique retarder) comprising a polymerized reactive liquid crystal thin film and a rubbed polymer alignment layer incorporating a polyimide having bulky side-chain groups (i.e., sterically hindered side-chain groups) is described. The alignment layer is produced by solvent casting a thin film of polyimide material containing bulky side-chains onto a substrate. The polyimide surface is then mechanically buffed as in conventional processing of liquid crystal displays. Next, a thin film of reactive liquid crystal material is deposited onto the polyimide alignment layer using a solvent casting technique. The buffed polyimide surface aligns the adjacent reactive liquid crystal molecules with a solid/nematic pretilt angle of between approximately 25xc2x0 and approximately 65xc2x0. Finally, the solvent is evaporated and the reactive liquid crystal film is photopolymerized (with ultraviolet light) so as to freeze in the film""s nematic order. In an alternative embodiment, the liquid crystal thin film incorporates a side-chain liquid crystal polymer. The large, uniform, pretilt angles achievable with the method are relatively insensitive to normal variations in processing conditions such as alignment layer rubbing strength, solvent exposure and drying temperature, polymerization temperature and the like.